th400 speedometer gear help

wes104

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oklahoma
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wesley
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1989
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r3500
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454
i have a 89 gmc r3500 with a th400 i just rebuilt and noticed that the speedometer is accurate to about 55 than at about 60 its 2-3 miles slower and at 65 its about 3 to 5mph slower. it currently has a 16 tooth drive gear and 40 tooth driven gear in it.i was wondering if going to a 34-39 housing and 39 tooth gear fix that?
 

AuroraGirl

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Taylor
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i have a 89 gmc r3500 with a th400 i just rebuilt and noticed that the speedometer is accurate to about 55 than at about 60 its 2-3 miles slower and at 65 its about 3 to 5mph slower. it currently has a 16 tooth drive gear and 40 tooth driven gear in it.i was wondering if going to a 34-39 housing and 39 tooth gear fix that?
sounds like a speedo or cable issue unless the gear is worn visibly, i would inspect it and your cable. if its accurate TOO 55 that tells me something is working right enough for quite a bit
 

Matt69olds

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455 Olds
I would agree. The speedo is calibrated with a specific drive/driven gear depending on tire diameter and rear end ratio. It should be accurate at any speed if the gears are correct. Sound like the speedo itself is the problem.
 

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